A Discussion of Traditional Rock Slope Stability Analysis Methods with a Technical Evaluation of a Former Quarry Site in Jessup, Pennsylvania

Abstract

ABSTRACT: Rock slope stability is generally controlled by the orientations of the discontinuities present in a rock mass. Depending upon the orientations of these features and the orientation of the slope in question, slope failures may be possible. In this analysis, common analytical methods are discussed and presented for a site in Pennsylvania. An analysis first begins with discontinuity mapping. In this study, quarry walls were mapped, and observations of active slope failures were made. The data collected were plotted on stereonets for cluster and kinematic analyses, which identified several potential failures. Field and laboratory data were used to calculate the Factor of Safety for each predicted failure mode for each slope. Several slopes had elevated failure risks and mitigation recommendations were provided to the client. Rock slope stability analysis is a crucial step when considering development in proximity to rock slopes. It is necessary to understand the continuity of the rock mass and failure risks, and to mitigate those risks which can have devastating impacts on human lives and infrastructure. This technical paper aims to present a basic example of a complete analysis in the hope of providing a guide for fundamental rock slope stability analysis methods. 1. INTRODUCTION Rock slope stability at a site is generally controlled by the orientations and conditions of the discontinuities (bedding planes, fractures, faults, and joints) that are present in the rock mass. These roughly planar features often form the edges of the individual blocks of rock that constitute the overall rock mass. Depending upon the orientations of these features and the orientation and dip of the slope in question, planar, wedge, and/or toppling failures may be indicated. Other factors such as the presence of groundwater, vegetation, ice, surcharge loads, seismicity, along with the geomechanical properties of the rock can also greatly affect the stability of rock slopes. 2. BASIC FIELD METHODS To understand the fabric of the rock mass and determine the orientations of the discontinuities present throughout, a geologist or geotechnical engineer should visit the site of the proposed or existing rock slope to complete a thorough reconnaissance of the slope(s) in question. The reconnaissance should involve a systematic evaluation of rock slope conditions including detailed rock discontinuity mapping, observation, examinations of the presence of groundwater and photo-documentation of key rock slope features.